What's behind Titan's mysterious bright 'magic island'?

Eight years of Cassini photography has uncovered a strange region brightening and growing on Saturn's largest moon. Belinda Smith reports.

Changes in the 'magic island', a feature in a hydrocarbon sea on Saturn's moon Titan, is likely caused by waves, scientists think. – NASA/JPL-Caltech/ASI/Cornell

Saturn's largest moon Titan is a world of rocks, ice and vast hydrocarbon lakes ... so why is a patch jutting out into a hydrocarbon lake growing and shrinking?

Radar photos taken by the Cassini-Huygens mission, flying past the moon since 2004, shows the moon's surfact as a dynamic system. Transient features arise and melt back into the surface over the course of years.

The series of photos above show the shape and size of what's dubbed the "magic island" in Ligeia Mare, the moon's second-largest hydrocarbon sea. With an area of around 130,000 square kilometres, Ligeia Mare is half as big again as Lake Superior in North America.

Since Cassini-Huygens has circled the moon, planetary scientists noticed something amiss in region, with one segment appearing to grow, brighten and shrink. The series of 500-kilometre-wide insets of the image above shows the "magic island" and its eight-year evolution.

Other smaller bright spots have blossomed and faded over the course of Cassini's orbits in Ligeia Mare, as well as the largest hydrocarbon sea on Titan, Kraken Mare.

So what's going on?

While scientists called Ligeia Mare's strange spot an "island", the region is actually part of the hydrocarbon lake's surface. The brightening, they think, is due to waves on the lake's surface reflecting radar back, like a mirror tilted on just the right angle.

Other theories are changes in the solids floating on or just below the lake's surface, reflecting or absorbing the radar as its bounced off the moon.

We'll be able to see the next instalment soon: Cassini is due for another flyby in 2017, and will snap more photos as it zooms by.